1. Field of the Invention
The present invention relates generally to asynchronous transfer mode (ATM) switching systems, and more specifically the invention relates to monitoring the quality of ATM cells forwarded from a buffer according to their cell loss rate.
2. Description of the Related Art
In ATM switching systems, user traffic is monitored by a mechanism known as Usage Parameter Control (UPC) to determine if the user traffic violates the contract agreed upon between the user and the network. If the user traffic violates the contract or congestion occurs in the network traffic, user""s cells are discarded. The discarded cells are monitored to determine the cell loss rate of the user traffic. The cell loss rate is checked against the quality-of-service (QoS) parameter requested by the user to determine if it satisfies the requested QoS parameter. If the user""s QoS parameter is not satisfied, the restriction imposed on the user traffic is relaxed according to the cell loss rate.
Two prior art techniques are available to monitor the quality of ATM cells, i.e., the jumping window cell-loss-rate monitor and the sliding window cell-loss-rate monitor.
While the jumping window type is simpler to implement than the sliding window type, the performance of the jumping type monitor largely depends on the pattern of occurrences of discarded cells because of a fixed time window. The sliding window type monitor eliminates the drawback of the jumping window type monitor by using a number of window defining circuits that respond cyclically to discarded cells to define a number of sliding time windows. However, the sliding window type monitor is not simple to implement.
It is therefore an object of the present invention to provide a quality monitor apparatus and method that is simple to implement and eliminates the drawback of the prior art jumping window type monitor.
According to a first aspect of the present invention, there is provided a quality monitoring apparatus for monitoring the quality of ATM cells stored in a buffer, comprising window defining circuitry for incrementing a count value by a predetermined value in response to each cell discarded from the buffer and decrementing the count value by one in response to each cell forwarded from the buffer to define a time window of variable size, a cell loss counter for incrementing a count value in response to each cell discarded from the buffer within the time window, and decision circuitry for producing a low quality signal when the count value of the cell loss counter is greater than a threshold value.
According to a second aspect, the present invention provides a method of monitoring the quality of ATM cells stored in a buffer, comprising the steps of (a) incrementing a first count value by one in response to a cell discarded from the buffer, (b) incrementing a second count value by a predetermined value in response to the discarded cell, (c) decrementing the second count value by one in response to a cell forwarded from the buffer, (d) determining if the second count value is equal to zero, (e) if the second count value is not equal to zero, repeating the steps (a) to (d), and if the second count value is equal to zero, resetting the first count value to zero and repeating the steps (a) to (d), (f) determining if the first count value is greater than a threshold value, and (g) if the first count value is greater than the threshold value, producing a low quality signal and repeating the steps (a) to (f) and if the first count value is not greater than the threshold value, repeating the steps (a) to (f).
According to a third aspect, the present invention provides an ATM switching system comprising a self-routing switch, a cell buffer connected to the self-routing switch, the cell buffer being partitioned into a plurality of storage locations corresponding to different service classes, address control circuitry connected to the self-routing switch for producing an address signal associating each ATM cell delivered from the self-routing switch with one of the storage locations and storing the ATM cell into, and reading a stored ATM cell out of, the associated storage location, a plurality of quality monitors corresponding respectively to the storage locations of the cell buffer; and bandwidth control circuitry for controlling the storage locations of the cell buffer according to output signals of the quality monitors. Each of the quality monitors includes window defining circuitry for incrementing a count value by a predetermined value in response to each cell discarded from a corresponding storage location of the cell buffer and decrementing the count value by one in response to each cell forwarded from the corresponding storage location to define a time window of variable size, a cell loss counter for incrementing a count value in response to each cell discarded from the corresponding storage location within the time window, and decision circuitry for producing a low quality signal as an output signal of the quality monitor when the count value of the cell loss counter is greater than a threshold value.